Register threads with `Masstree` / unlock excess cache entries in scans

[mmarkakis]

This PR fixes two bugs in the current master:

• Foreground threads need to be registered with Masstree.
• Any cache entries that were retrieved as part of a scan, but were not eventually returned to the caller, need to be unlocked before returning.

After applying these fixes on gy-pg-rw-sweep, calling cond run //scripts/rw_sweep:rw_sweep-pg_llsm-64-0-zipfian runs to completion in under 2 minutes.

cc: @andreaskipf @geoffxy